The invention relates generally to a DC power circuit in hybrid and electric vehicles and, more particularly, to a system and method for leakage current detection and fault location identification in a DC power circuit in hybrid and electric vehicles.
Hybrid electric vehicles may combine an internal combustion engine and an electric motor powered by an energy storage device, such as a traction battery, to propel the vehicle. Such a combination may increase overall fuel efficiency by enabling the combustion engine and the electric motor to each operate in respective ranges of increased efficiency. Electric motors, for example, may be efficient at accelerating from a standing start, while combustion engines may be efficient during sustained periods of constant engine operation, such as in highway driving. Having an electric motor to boost initial acceleration allows combustion engines in hybrid electric vehicles to be smaller and more fuel efficient.
Purely electric vehicles use stored electrical energy to power an electric motor, which propels the vehicle and may also operate auxiliary drives. Purely electric vehicles may use one or more sources of stored electrical energy. For example, a first source of stored electrical energy may be used to provide longer-lasting energy while a second source of stored electrical energy may be used to provide higher-power energy for, for example, acceleration.
Plug-in electric vehicles, whether of the hybrid electric type or of the purely electric type, are configured to use electrical energy from an external source to recharge the traction battery. Such vehicles may include on-road and off-road vehicles, golf cars, neighborhood electric vehicles, forklifts, and utility trucks as examples. These vehicles may use either off-board stationary battery chargers or on-board battery chargers to transfer electrical energy from a utility grid or renewable energy source to the vehicle's on-board traction battery. Plug-in vehicles may include circuitry and connections to facilitate the recharging of the traction battery from the utility grid or other external source, for example. Additionally, an export power inverter may be provided that is able to invert power received from the DC bus of the vehicle and output an AC power that may be provided back to the utility grid or to another AC load that might require power.
The DC power circuit of the vehicle—i.e., the energy storage device(s), battery charger, export inverter and traction motor or other loads connected to a DC bus/network in the vehicle—are generally operated such that they are electrically isolated from the vehicle frame, such that a fault (short circuit) between one of the DC power conductors and the vehicle frame does not produce large fault currents. While such protection is provided due to the isolation of the DC power circuit from the vehicle frame, it is recognized that it is desirable to detect high impedance, low leakage current faults in the DC power circuit during operation of the vehicle, such that a protection strategy is provided to detect a fault and turn off power in the DC power circuit before a second fault develops. Furthermore, in the case that the fault is a leakage current through a person, it is desired to limit the amount of current needed to detect the fault to low levels.
An example of ground fault circuitry as known in the art for detecting leakage current to a vehicle frame as presented by TDI Power, Inc. is shown in FIG. 1, where a vehicle DC power circuit 2 is illustrated that includes ground fault detection circuitry 4 and associated resistors 6 that functions to detect leakage current to the vehicle frame that arises from a fault anywhere on the DC power bus 8. The ground fault detection circuitry 4 functions to detect a voltage change across the resistor(s) 6 that is indicative of a leakage current in the DC power circuit 2. The actual voltage during a fault depends on the ratio of the resistance of the fault leakage path to the resistance of the two biasing resistors 6. For the values of the resistors 6 shown in FIG. 1, the ratio of the resistance of the usual causes of leakage current to the biasing resistance is low enough to produce a significant change in voltage during a fault. During normal conditions, the resistors 6 establish a voltage of 95 volts across the 1 Megaohm resistor. During a fault from positive supply to ground, the voltage across the 1 Megaohm resistor can rise to as high as 380 volts. During a fault from negative supply to ground, the voltage can drop to as low as 0 volts. In the case of the circuit 2 in FIG. 1, the current through a fault from the positive side of the DC bus 8 to the vehicle frame is limited to +0.38 milliamps. For a fault from the negative side of the DC bus 8 to the vehicle frame, the fault current is limited to approximately −0.13 milliamps.
However, while the ground fault detection circuitry of FIG. 1 can detect leakage current to the vehicle frame that arises from a fault anywhere on the DC power bus, the ground fault detection circuitry does not give any indication concerning the location of the fault. That is, the voltage readings acquired/analyzed by the ground fault detection circuitry provides no information on the location of the fault within the vehicle DC power circuit, as the voltage readings are not dependent on the location of the fault. It is recognized that an indication of the location of the fault would be beneficial, in that such information would be useful for diagnostics and repair of the DC power circuit.
It would therefore be desirable to provide a system and method for leakage current detection and fault location identification in a DC power circuit in hybrid and electric vehicles. Such a system and method would identify the location of a fault in the DC power circuit, so as to provide for diagnostics and repair of the DC power circuit. The system and method could be employed in a stand-alone fashion without use of other known fault detection circuitry, or could also be used in combination with other known fault detection circuitry.